Variable optical attenuators (VOAs) are key components of optical networks, including local and long distance telephone networks. High performance telecommunication systems of the future are expected to rely on VOAs to perform a variety of functions such as filtering, switching, splitting, coupling, and combining as well as to participate in other functions such as multiplexing and demultiplexing.
Optical signal attenuation can be accomplished in a variety of ways through diverting all or a portion of an optical signal from an original pathway. The diverted optical signal can be discarded for performing such operations as in filtering or can be transferred to one or more additional pathways for performing such operations as switching, splitting, and coupling. In addition, optical signals traveling in different original pathways can be diverted in whole or in part into a common pathway, such as in combining operations.
The "variable" capability of VOAs distinguishes them from simpler discrete state devices, such as on-off switches, that direct all or none of the optical signal to selected pathways. A range of operating positions within VOAs provides a further capability to distribute or collect optical signals in different proportions among pathways.
U.S. Pat. No. 5,146,519 to Miller et al. and U.S. Pat. No. 5,353,363 to Keck et al. disclose VOAs using fiber couplers to provide the variable attenuating function. The fiber couplers include one or more fibers within an overcladding that are drawn down to a reduced diameter at a midsection. Bending or twisting about the midsection of the coupler changes the optical characteristics (e.g., propagation constant) of the coupler to progressively couple or uncouple light between adjacent fibers or change modes of transmission within a single fiber.
Correlating a desired amount of attenuation with a command signal presents difficult mechanical challenges because of a high sensitivity of the fiber couplers to bending. In addition, friction and backlash among moving elements of a bending actuator and thermal size variations of the actuator and fused coupler mountings undermine repeatability.